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Line (edge) constraint

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A line constraint, also known as an edge constraint, may be applied to the edge of a shell or solid object. When applied along an edge, the line constraint will constrain all interconnecting objects to the joints selected. These joints will then displace, along those DOF selected, as a function of interpolation between the two master joints which govern constraint behavior.

NOTE: Slight displacement discontinuity should be expected along any edge constraint. This discontinuity is sufficiently small relative to other deformations.

NOTE: When an edge constraint is assigned to a series of joints, and some of those joints fall within the auto-merge tolerance of the end nodes, a rigid-body constraint is applied to avoid ill-conditioning within the stiffness matrix. This may cause coupling within the constraint, leading to unintended behavior.

Example

An example edge constraint is shown in Figure 1: Figure 1 - Edge constraint along shell element

Here, two edge constraints are applied at each end of a shell object which simulates the span of a bridge deck. Each of the four edge constraints has a master joint at either end, and a dependent joint at the intermediate-column intersection. If the joints circled in Figure 1 are numbered, from left to right, 1, 2, 3, 4, 5, then joint 2 is governed by masters 1 and 3, and joint 4 is governed by masters 3 and 5. Given this edge-constraint condition, each dependent-joint displacement is then interpolated from the displacement of the two master joints.

To apply an edge constraint, first select the dependent joint and assign it to the line constraint. Then select the two master joints and assign them to the same line constraint. Equal constraints may also be useful to constrain objects to edge-constraint joints. From the previous example, equal constraints may constrain the corners of the bridge superstructure to the top of each column.

Using weld constraints may sometimes be more effective and productive than using edge constraints. For example, the bridge-deck model shown in Figure 1 may be more efficiently modeled through the following process:

1. Divide the two shells along the intermediate columns to create four shell objects.
2. Use edge constraints to transition the mesh further inside the domain.
3. Disconnect the top of each column from the shells, producing eight joints.
4. Select these eight joints and assign them to a single weld constraint.